1. Field of the Invention
The present invention relates to an active matrix substrate and a liquid crystal display apparatus, and to a method for producing the same. More particularly, the present invention relates to a liquid crystal display apparatus having excellent continuity of contact holes and excellent image characteristics and to an active matrix substrate capable of realizing such a liquid crystal display apparatus, and to a method for producing the same.
2. Description of the Related Art
Recently, the application of a liquid crystal display apparatus to a word processor, a lap-top personal computer, a pocket TV set, etc. has been increasing rapidly. Liquid crystal display apparatuses of the reflection type have been particularly attracting much attention since they display an image by reflecting the incident light from outside and do not require a back light. As a result, the power consumption is low and the apparatus can be made to be thin and light-weight.
Conventionally, TN (Twisted Nematic) mode and STN (Super Twisted Nematic) mode are employed in the reflection type liquid crystal display apparatus. Since these modes require a polarizing plate, one half of the light intensity of the natural light is inevitably not used for display and, therefore, the display becomes dark.
In order to overcome this problem, display modes in which all of the natural light beams are effectively used have been suggested. An example of such display modes is phase transition type guest-host mode (D. L. White and G. N. Taylor: J. Appl. Phys. Vol. 45, pp. 4718, 1974; referred to as White publication hereinafter). In this display mode, a cholesteric-nematic phase transition phenomenon by electric field is used. Also suggested is a reflection type multi-color display where micro color filters are incorporated in the phase transition type guest-host mode (for example, refer to Tohru Koizumi and Tatsuo Uchida. Proceedings of the SID. Vol. 29/2, pp. 157. 1988).
In order to obtain a brighter display in such display modes which do not require the polarizing plate, it is necessary to increase the intensity of the incident light scattering in the direction perpendicular to the display screen for all incident angles. To accomplish such, it is necessary to produce a reflecting plate having optimal reflecting characteristics. In the above-mentioned White publication, a description is provided to obtain such a reflecting plate as follows. The surface of a substrate made of glass or the like is roughed by a grinding agent. Then, after a certain period of time, the substrate is etched via hydrofluoric acid to form an uneven surface, and then a thin film of silver is formed on the uneven surface.
However, since the uneven surface is formed by scraping the glass substrate with the grinding agent, it is difficult to form a uniform uneven surface. Reproducibility in consistently forming the uneven surface is also poor.
FIG. 21A is a plan view of a matrix substrate 2 having thin film transistors 1 (referred to as TFT hereinafter) which are switching devices used in the active matrix mode, and FIG. 21B is a cross-sectional view of the matrix substrate 2 illustrated in FIG. 21A taken along the B--B line. The matrix substrate includes a plurality of gate bus lines 3 made of chromium, tantalum or the like provided in parallel to each other on the insulating matrix substrate 2 made of glass or the like, and a gate electrode 4 which is branched from the gate bus line 3. The gate bus line 3 functions as a scanning line.
As illustrated in FIG. 21B, a gate insulating film 5 made of silicon nitride (SiN.sub.x), silicon oxide (SiO.sub.x ) or the like is formed on the entire surface of the substrate 2a, covering the gate electrode 4. Formed on the portion of the gate insulating film 5 on the gate electrode 4 is a semiconductor layer 6 which is made of amorphous silicon (referred to as a-Si hereinafter), polycrystalline silicon, CdSe, etc. Formed on both sides of the semiconductor layer 6 are n.sup.+ - or p.sup.+ -contact layers 11 made of a-Si, polycrystalline silicon, CdSe, etc. Furthermore, as illustrated in FIG. 22, the gate insulating film 5 is formed on the entire surface of the substrate 2a except the portions on the input terminals 3a of the gate bus lines 3.
As illustrated in FIG. 21B, a source electrode 7 made of titanium, molybdenum, aluminum, etc. is formed and stacked on one side of the semiconductor layer 6. Formed and stacked on the other side of the semiconductor layer 6 is a drain electrode 8 which is also made of titanium, molybdenum, aluminum, etc. in a similar manner as the source electrode 7. A pixel electrode 9 made of a transparent conductive film such as ITO (Indium Tin Oxide) is formed and stacked at the edge of the drain electrode 8 opposite to the semiconductor layer 6.
As illustrated in FIGS. 21A and 21B, a source bus line 10 which crosses the gate bus line 3 with the gate insulating film 5 interposed therebetween is connected to the source electrode 7. The source bus line 10 functions as a signal line. The source bus line 10 is also formed of a similar metal as the source electrode 7. The gate electrode 4, the gate insulating film 5, the semiconductor layer 6, the source electrode 7 and the drain electrode 8 constitute the TFT 1, which has a function as a switching device.
When the matrix substrate 2 having TFTs 1 as illustrated in FIGS. 21A, 21B and 22 is applied to a reflection type liquid crystal display apparatus, it is necessary to form the pixel electrodes 9 of a metal having a light reflecting property such as aluminum, silver, etc. and to form the uneven surface on the gate insulating film 5. Generally, it is not desirable to form the uneven surface on the gate insulating film 5 since it has negative effects on the device forming processes. Furthermore, it is difficult to uniformly form the tapered uneven surface on the insulating film 5 which is made of an inorganic material.
Japanese Laid-Open Patent Publication No. 56-94386 to Yazawa et al. (referred to as Yazawa publication 1 hereinafter) discloses a method for increasing the intensity of light scattering in the direction perpendicular to the display screen, where a metal thin film layer having an uneven surface is used as a reflecting plate of a liquid crystal display apparatus, and also describes methods for producing the metal thin film layer listed as (1), (2) and (3) below.
(1) A method where a metal thin film layer is formed on the substrate by evaporation or sputtering under a particular condition, and a metal thin film having an uneven surface is obtained.
(2) A method where a metal thin film layer formed on the substrate by evaporation or sputtering is subjected to heat treatment and recrystallization to obtain a metal thin film layer having an uneven surface. For example, when aluminum or aluminum alloy is used as a material for the metal thin film layer, since the melting point of the material is 660.degree. C., the recrystallization is carried out in the temperature range of 100.degree. C. to 600.degree. C. This recrystalization is responsible for the rearrangement of atoms within the metal thin film, which results in the metal thin film layer having an uneven surface.
(3) A method where, as illustrated in FIG. 23, an alloy thin film layer 63 formed on the substrate 2 by evaporation or sputtering is subjected to heat treatment so that precipitations 64 precipitate, and then the portion of the alloy thin film layer 63 proximate to the surface is removed by etching. For example, when the alloy thin film layer 63 obtained by mixing 2% weight of silicon in aluminum is heated in a N.sub.2 environment at 400.degree. C. for 20 minutes, an intermetallic compound of aluminum and silicon having a particle diameter of about 0.2 to 1.0 .mu.m precipitates as the precipitation 64. For example, when the alloy thin film layer 63 of 1.0 .mu.m thickness is subjected to precipitation treatment and then the 0.2 .mu.m portion from the surface of the layer is removed by etching, the surface becomes white.
Also described in Yazawa publication 1 is that the surface of the metal thin film layer can be treated by sand blasting. Further described in the publication is that since the uneven surface and the steps on the surface of the metal thin film layer have some negative effect when forming an aligning treatment film, a transparent thin film such as an organic thin film made of a silicone resin, an epoxy resin, a polyimide resin or the like or an inorganic resin can be formed on the surface of the liquid crystal driving electrodes (pixel electrodes) so as to flatten the surface, thereby enhancing the effect of the aligning treatment.
However, the method for forming the reflecting plate disclosed in the above-mentioned Yazawa publication 1 depends largely on a chance factor. Like the reflecting plate described in the above-mentioned White publication (the formation of the reflecting plate includes roughening the surface of the substrate made of glass or the like by the grinding agent, etching the substrate via hydrofluoric acid after a certain period of time to form the uneven surface, and forming the thin film of silver on the uneven surface), it is difficult to uniformly form the tapered unevenness.
Furthermore, the fact that the reflecting plate is white means that light reflecting on the reflecting plate scatters in all directions. When the reflecting plate also functions as the liquid crystal driving electrode of the liquid crystal display apparatus and is formed on the substrate surface which is in contact with the liquid crystal, light from the reflecting plate gets out to the atmosphere through the liquid crystal layer and the opposing substrate. When the refractive index of the liquid crystal layer and the substrate is assumed to be 1.5 and the refractive index of the air to be 1, if the scattered light from the reflecting plate is incident on the interface between the atmosphere and the substrate with an angle from the vertical of more than about 48.degree., then the scattered light is reflected at the interface and cannot get outside of the liquid crystal display apparatus. Therefore, if such a reflecting plate is used, the portion of the dispersed light which can be used as the light for display becomes limited and the display screen becomes dark. Therefore, in order to obtain a brighter display screen, it is necessary that the reflecting plate has directionality so that it can control the scattering angle of the reflected light. However, as described above, it is considerably difficult to control the scattering angle of the reflected light from the reflecting plate in the methods described in Yazawa publication 1 and White publication because of the chance factor on which the formation of the reflecting plate largely depends. It is also difficult to achieve better reproducibility.
Japanese Laid-Open Patent Publication No. 56-156864 to Yazawa et al. (referred to as Yazawa publication 2 hereinafter) describes with regards to the reflection characteristics of the reflecting plate formed by heating aluminum or aluminum alloy, that even if the reflecting plate formed by subjecting the aluminum to heat treatment in an inert gas environment or a hydrogen gas environment at 400.degree. C. to 450.degree. C. is used, since the ratio of the mirror surface portion is large, the entire display panel becomes dark. Consequently, in order to increase the intensity of the light scattering in the desired direction, it is necessary to perform the heat treatment at higher temperatures. However, such heat treatment is not desirable because it destroys the TFT devices or MIM devices used as switching devices (for example, in the case of a-Si*TFT, dehydrogenation in the semiconductor layer begins at 350.degree. C.).
Yazawa publication 2 describes a method for forming a reflecting plate at a low temperature having directionality which controls the scattering angle of the reflected light, so that it can be used in an active matrix type liquid crystal display apparatus where a-Si*TFT devices or MIM devices are used as switching devices. According to this method, SiO.sub.2 is first formed on the substrate surface in a triangular wave-shape by CVD, and then aluminum is deposited thereon to form a reflecting plate 65 as illustrated in FIG. 24. The reflecting plate 65 has a cross-section of a near sinusoidal wave-shape with the mean slope angle of .theta.=5.degree. to 30.degree..
Japanese Laid-Open Patent Publication No. 56-156865 to Yazawa et al. (referred to as Yazawa publication 3 hereinafter) describes that when a reflecting plate obtained by subjecting the aluminum or the aluminum alloy to heat treatment and then optionally by etching is used in the liquid crystal display apparatus, the display characteristics of the display apparatus become degraded. In order to cope with this problem, it is disclosed in Yazawa publication 3 that the reflecting plate 66 having the uneven surface as illustrated in FIG. 25 is formed by performing taper-etching on the SiO.sub.2 formed by CVD in a triangular wave-shape and then by depositing aluminum thereon.
To form a reflecting plate by forming a metal thin film on an insulating resin layer having an uneven surface, the method described below has been suggested.
U.S. Pat. No. 4,519,678 to Komatsubara et al. (referred to as Komatsubara publication hereinafter) discloses the following method. First, a polymer based (for example, polyimide based) resin is applied onto the substrate having devices formed thereon and then the resin is thermally cured to form a resin layer. Then, a resist pattern is formed thereon by photolithography, which is then used as a mask when performing wet-etching or dry-etching (RIE, etc.) so that indentations are formed on the resin layer. Then, after removing the resist pattern, the resin layer is heated at 150.degree. C. to 500.degree. C. so as to smooth the edges, peaks, and/or valleys of the indentation. After forming the uneven surface as described above which has a smooth cross-section and further forming a contact hole by photolithography, aluminum is deposited on the resin layer by vacuum evaporation, thereby forming the reflecting plate.
Another method is also described in the Komatsubara publication. A plurality of cylinder-shape protrusions are formed on the substrate and then a resin layer is applied thereon and cured so that the resin layer of which the cross-section thereof has a smooth uneven surface is formed. Then, a reflecting plate made of aluminum, silver, an alloy thereof or the like are formed on the resin layer. The above-mentioned cylinder-shape protrusion is formed by forming on the substrate a single layer or a plurality of layers of insulator, semiconductor or metal and by selectively etching the layer(s) using a mask pattern (a resist). In either case, the metal thin film which is formed on the resin layer is in electrical contact with the electrode on the substrate via the contact hole formed in the pesin layer.
Japanese Laid-Open Patent Publication No. 6-75238 to Nakamura et al. describes that a reflecting plate made of a metal thin film is formed on an insulating film by applying a photosensitive resin on the substrate, by exposing and developing the photosensitive resin with light blocking means including circular light blocking regions, and by performing heat treatment so that a plurality of protrusions are formed, and then by forming an insulating film on the plurality of protrusions along the protrusion-shape of the protrusions.
As described above, it is preferable to produce the reflecting plate by forming a desired uneven surface with an insulating film interposed therebetween, which is far from the devices (close to the interface with the liquid crystal), so that the devices formed on the substrate are not affected. When producing the reflecting plate, it is more desirable to form the insulating layer having an uneven surface under the metal thin film and then to form the metal thin film in a mirror-surface condition along the uneven surface than to treat (heat treatment, etching, etc.) the surface of the metal thin film itself to be used as the reflecting plate. Furthermore, since it is difficult to realize a uniform uneven surface having a smooth cross-section in the insulating layer made of an inorganic material, it is preferable to form the insulating layer by using a resin which can easily be controlled for producing the uneven surface.
When a resin layer is used to form a reflecting plate having desirable reflection characteristics (directionality), it is necessary to form a contact hole which connects the reflecting plate (reflecting electrode) formed on the resin layer to the device (switching device, etc.) formed under the resin layer. The reflecting electrode is connected via the contact hole to a drawing-out electrode extending from the device formed on the substrate. The drawing-out electrode mentioned here is an electrode for applying voltage for displaying to each reflecting electrode.
The contact hole is formed by photolithography after thermally curing the resin layer when a non-photosensitive resin is to be used. When a photosensitive resin is to be used, it is formed when the resin layer is formed by exposure and development. It is advantageous to use the photosensitive resin because of the fewer number of steps.
When the resin layer is formed of a photosensitive resin, the thickness of the resin layer decreases due to the development even if it is the portion to be selectively left by exposure and development. Therefore, the effect of the development time when forming the uneven surface having smooth cross-section on the resin layer is large. On the other hand, the longer the development time, the better the continuity of the contact hole becomes.
The developing speed for the resin layer within the substrate is faster on the periphery of the substrate than in the central region. Therefore, when the reflecting plate is formed by using a large substrate of 300.times.300 mm or greater (e.g., if the development is performed for a sufficient time to obtain excellent continuity of a contact hole in the central region of the substrate), then the periphery of the substrate becomes excessively developed and the reflecting plate becomes more of a mirror-like condition. For this reason, a liquid crystal display apparatus utilizing the reflecting plate formed at the peripheral portion of the substrate tends to have a darker display.
When the contact hole is formed by dry-etching, for example, by RIE, using a non-photosensitive resin such as the one described on the above-mentioned Komatsubara publication, because the plasma of the dry-etching is of high density in the central region of the substrate, the etching proceeds from the center of the substrate. Since both the resist which is used as a mask pattern and the resin are an organic film, it is difficult to have the selection ratio thereof to be greater than 10. Therefore, if the etching is carried out in such a manner that sufficient continuity of the contact hole is obtained on the peripheral region of the substrate, then the resist is excessively etched in the central region of the substrate and, as a result, the contact hole becomes too large and the film thickness of the resin decreases. For this reason, a liquid crystal display apparatus utilizing the reflecting plate formed in the central region of the substrate has darker display.
Moreover, the thickness of the liquid crystal layer in a liquid crystal display apparatus and the gap between a pair of substrates before injecting liquid crystal (referred to as the cell gap hereinafter) are important parameters affecting the response time, the contrast, etc. of a liquid crystal display apparatus. Therefore, it is important in view of production control of liquid crystal display apparatuses to measure the thickness of the liquid crystal layer, the cell gap, etc.
Conventionally, in the case of a transmission type liquid crystal display apparatus or a reflection type liquid crystal display apparatus having the reflecting plate on the outside of a pair of substrates, the cell gap was measured by using the interference of two kinds of reflected light with the liquid crystal cell being in a transmission condition, one from the interface between the alignment film of one of the substrate and the liquid crystal layer or the air layer and the other from the interface between the liquid crystal layer or the air layer and the alignment film of the other substrate. However, in a reflection type liquid crystal display apparatus having the reflecting plate formed inside of a pair of substrate as a pixel electrode, since the intensity of the scattered reflection light from the reflecting electrode is too large, the measurement of the wavelength of the interference light is difficult and the conventional measurement method utilizing the interference of light cannot be used.
There is another method for measuring the cell gap which uses laser light. This method, the outline of which is illustrated in FIG. 26, uses an optical system including a first lens 17a which collimates laser light from the semiconductor laser 19 into parallel light and a second lens 17b which collects laser light reflected at the sample 18 to be measured. The method is based on the fact that when the focal point of the second lens 17b is on the reflection surface (e.g., the interface between the alignment film and the liquid crystal layer, or the surface of the reflecting electrode, etc.) of the sample 18, the reflected light which is fed back to the system reaches its peak. The locations of the second lens 17b corresponding to the peaks of the two reflected lights (i.e., the distance traveled by the lens) can be used to calculate the interval between the two reflection surfaces.
FIG. 27 illustrates the result of the measurement by this method made on the surface of the reflecting electrode having the uneven surface formed thereon to obtain desired scattered light. As can be seen from FIG. 27, since the laser light scatters at the reflecting electrode having the uneven surface, the peak of the reflected light cannot be obtained. Therefore, the location of the reflecting plate cannot be determined by the measurement using laser light, which makes the measurement of the cell gap impossible.
The formation of the contact hole and its continuity are an important problem in the transmission type liquid crystal display apparatus as well as in the reflection type liquid crystal display apparatus. Problems associated with conventional liquid crystal display apparatuses will further be described.
FIG. 28 is a schematic cross-sectional view of a conventional liquid crystal display apparatus 200, and FIG. 29 is a schematic plan view of an active matrix substrate 201 of the liquid crystal display apparatus 200 illustrated in FIG. 28. As illustrated in FIG. 28, the liquid crystal display apparatus 200 includes an active matrix substrate 201, an opposing substrate 202 and a liquid crystal layer 35 held between the two substrates. The active matrix substrate 201 includes a glass substrate 21, and driving signal lines 30, pixel electrodes 27 and driving devices 29 (switching devices) formed on the glass substrate 21, and the opposing substrate 202 includes a substrate 38, and an opposing electrode 36 and color filters 37 formed on the substrate 38.
As illustrated in FIG. 29, the pixel electrodes 27 are arranged in a matrix configuration on the active matrix substrate 201, and each pixel electrode 27 is connected to the driving signal line 30 via a corresponding driving device 29. This liquid crystal display apparatus 200 uses a diode type two-terminal device as the driving device 29.
FIG. 30A is a schematic plan view illustrating a region of the active matrix substrate 201 corresponding to one pixel, and FIG. 30B is a schematic cross-sectional view of the substrate in FIG. 30A taken along the line B--B. With reference to FIGS. 30A and 30B, the structure proximate to the driving device 29 will be described.
As illustrated in FIG. 30A, the driving signal line 30 has a branch, and this branch serves as a lower electrode 22 of the driving device 29. As illustrated FIGS. 30A and 30B, the lower electrode 22 is formed on the glass substrate 21 at the location of the driving device 29, and an insulating film 23 is formed thereon. Patterned and formed on the insulating film 23 is an upper electrode 24. As described above, the diode type two-terminal device 29 having the lower electrode 22, the insulating film 23 and the upper electrode 24 is constructed. An insulating protective film 26 is formed on the entire surface of the glass substrate 21 so as to cover the driving devices 29. Patterned and formed on the insulating protective film 26 is the pixel electrode 27. The pixel electrode 27 is electrically connected to the upper electrode 24 of the driving device 29 via the contact hole 28 provided in the insulating protective film 26.
When an inorganic insulating film or a thermosetting resinous material is used for the insulating protective film 26, another process such as photolithography is necessary in order to form the contact hole 28 by patterning. When a photosensitive resinous material is used for the insulating protective film 26, the processes can be simplified because the contact hole 28 can be formed by exposure and development. However, when the insulating protective film 26 is formed of the photosensitive resinous material, if the development residue of the photosensitive resinous material occurs inside the contact hole 28, then a good contact cannot be obtained between the pixel electrode 27 and the upper electrode 24. If a pixel defect due to poor contact resulting from such development residue of the insulating protective film 26 occurs, a display quality of the liquid crystal display apparatus becomes deteriorated.
As described above, liquid crystal display apparatuses having excellent continuity of the contact hole and excellent image characteristics are in demand. Furthermore, as to a reflection type liquid crystal display apparatus having the reflecting plate formed inside the pair of substrates as the pixel electrode, a liquid crystal display apparatus capable of having the cell gap be measured and having excellent production efficiency as well as having excellent continuity of the contact hole (i.e., excellent image characteristics) is desired.